I. Field of the Invention
The present invention generally relates to a burst signal transmission system and, more particularly, to a signal control circuit suitable for controlling a modulated carrier signal having a fixed envelope, for example, an output waveform from a nonlinear amplifier used for an FM (frequency modulation) or PM (phase modulation) in a time division multiple access (TDM) or time division multiple (TDMA) communication system.
II. Description of the Prior Art
Conventionally, phase shift keying (PSK) is well-known as a modulation scheme of a TDMA or TDM communication system. There are two reasons why the PSK scheme is frequently used in the TDMA OR TDM communication system. First, since according to the PSK modulation, coherent detection is effected in a receiving end, transmission characteristics are better than those of any other modulation scheme. Second, since waveform-shaping of a baseband signal is effected by means of, for example, a Nyquist filter in favor of effective utilization of the frequency band, easy burst control can be performed when the system is constituted with linear components. According to the PSK modulation, a carrier wave is modulated by a baseband signal having a limited bandwidth so as to have an envelope with smoothed node portions. Therefore, the burst control can be effected at the node portions so that the spectrum spread due to carrier switching can be suppressed to some extent.
However, since the PSK scheme is of a linear type as described above, a linear amplifier must be used as a power amplifier. The linear amplifier has an efficiency as low as a few percent, and is not suitable for a high power communication system.
On the other hand, when a modulaton scheme such as FM or PM which provides a fixed envelope without node portions is employed, a class C amplifier can be used as a power amplifier, thereby readily obtaining a high power communication system. However, when the modulated carrier with the fixed envelope is burst-switched in response to a rectangular wave, frequency spectrum spread occurs which not only interfers the other channels but also causes interaction between transmission and reception waves, thus adversely affecting a receiver set of the system per se.